Method of treating silicon and devices containing pn junctions



United States Patent 3,290,180 METHOD OF TREATING SILICON AND DEVICESCONTAINING PN JUNCTIONS Stephen S. Baird, Richardson, and Clyde R.Fuller, Plano, Tex., assignors to Texas Instruments Incorporated,Dallas, Tex., a corporation of Delaware No Drawing. Continuation ofapplication Ser. No. 178,583, Mar. 9, 1962. This application Apr. 5,1965, Ser. No. 445,767

6 Claims. (Cl. 148-15) This is a continuation of our patent application,Serial No. 178,583, filed March 9, 1962 and now abandoned. Thisinvention relates to a method of treating silicon to stabilize theelectrical properties of silicon surfaces, and relates particularly toan improved method of cleaning and oxidizing the surfaces of silicondiodes and other silicon semiconductor devices containing PN junctions.The characteristics of semiconductor devices are, in part, dependentupon conditions existing at their surfaces. In junction type devices,the protection of the exposed portion of the rectifying barrier isespecially important.

Cleaning the surfaces of the device is very desirable because cleansurfaces enhance the result obtained from oxidation of the surfaces.Oxidation alone, however, is relatively ineffective in producing stablesurfaces unless they are cleaned before oxidation. For example, anyorganic material remaining on a surface at the time oxidation takesplace tends to short the device and cause discontinuities in the oxide.

The use of steam as an oxidizing medium provides several advantages overfree oxygen. When steam is used, no inversion layer is formed on the Nside because of the absence of free oxygen. With steam, the oxide growsmuch faster than with free oxygen. Possibly this occurs because of adifference in the oxidizing agent. Oxidation occurs at thesilicon-silicon dioxide interface, and the oxidizing agent, possibly OHions, provided by the steam are able to penetrate the oxide faster thanoxygen can and thus react at a faster rate. After the steam oxidation,the silicon devices are cooled in helium, thus avoiding free oxygen inthe entire process.

An object of the present invention is to provide a method for protectingthe junction of silicon diodes.

Another object is to provide a method of surface cleaning and oxidizingsilicon diodes that will reduce the leakage current and only slightlyreduce the breakdown voltage.

A further object of the invention is the use of sulfuric acid along withother cleaning agents to clean the surfaces of a diode preparatory tooxidation.

Other objects, features and advantages will be apparent from thefollowing detailed description, taken in connection with the appendedclaims.

A number of variations and advantages in the cleaning process may bedevised, but the preferred process herein after described mostadvantageously utilizes the cleaning and surface treatment required inthe pre-oxidation process.

The silicon diodes, comprised of mesa etched slices, are degreased in asoxhlet containing trichloroethylene, xylene or some other suitablesolvent for thirty minutes. The solvent is then removed by rinsing theslices in methyl alcohol and thereafter rinsing them in water. To insurethe complete removal of all organic material from the surface of eachslice, the slices are subsequently boiled in nitric acid for twentyminutes at 100 C. and after- Wards rinsed in running deionized water forabout five minutes. Surface treatment is finally accomplished byimmersing the slices for about twenty minutes in concentrated sulfuricacid, heated to 200 C. Most of the .sulfate which remains on the surfacefrom the action of sulfuric acid is then removed'by rinsing the slicesfor five minutes in running deionized water.

As above indicated, the water rinse does not'co'mpl'etel'y remove alltraces of the sulfate from the silicon wafer. Removal of the remainderis necessary as it would contaminate the surface and increase the oxygenabsorption at .the surface. Moreover, oxygen, being a P-type dopant,will tend to cause an inversion layer at the surface of the wafer, whilethe oxygen and the sulfate degrade the reverse characteristics of asilicon diode. Oxygen, principally, seems to contribute to high leakagewhile the sulfate tends to reduce the breakdown voltage. Results haveshown that the use of a method which eliminates the presence of sulfateand free oxygen before and during oxidation and cooling of silicondiodes markedly reduces leakage and only slightly reduces breakdownvoltage.

The remaining sulfate is removed from the slices in the followingmanner: The slices are first soaked in hydrofluoric acid for thirtyminutes. The acid that remains on the surfaces of the slices is thenremoved by rinsing the slices in Water for about five minutes, thenboiling them in deionized water twice for about fifteen minutes each,then rinsing them in water for about five minutes, then boiling them innitric acid for about thirty minutes, and finally rinsing them inrunning deionized water for about thirty minutes.

The wet slices, now ready for oxidation, are placed in quartz boats,loaded into the oxidation furnace under steam and oxidized for aboutfive and one-half hours at atmospheric pressure. When oxidation iscompleted, steam is replaced by an inert gas such as helium, the slicesare dried therein for about one hour at the oxidation temperature of1200 C. and then cooled to room temperature under helium.

By the practice of this invention the tendency of silicon to absorboxygen is reduced, and during the oxidation of the silicon with steamfree oxygen is avoided. This results in minimizing the breakdown voltagedegradation and lowering the leakage of the oxide protected junction.The oxide film coating the slice is approximately 20,000 Angstromsthick.

An example illustrating the improvement in leakage current and breakdownvoltage by the practice of the invention is shown in Table I, in whichresults are compared when either oxygen or sulfate or both are allowedto come into contact with the slices, or remain on the slices during theoperation of the method.

It is to be understood that while the method of the invention has beendescribed with reference to specific cleaning solutions and the lengthof time of their use, variations may be resorted to without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

What is claimed is:

1. The method of preparing a protected silicon PN junction whichcomprises the steps of degreasing the silicon slice intrichloroethylene, rinsing it in methyl alcohol and water, boiling itfor 20 minutes at C. in nitric acid, rinsing it in deionized water for 5minutes, heating it in concentrated sulfuric acid at 200 C. for 20minutes and rinsing in deionized running water for 5 Patented Dec. 6,1966 minutes, soaking in hydrofluoric acid for 30 minutes and rinsing inwater 5 minutes, boiling in deionized water twice for minutes andrinsing in water 5 minutes, boiling in nitric acid 30 minutes andrinsing in deionized running Water for 30 minutes, oxidizing about fiveand onehalf hours in steam at atmospheric pressure at 1200 C., drying inhelium for one hour and cooling to room temperature.

2. The method as described in claim 1 wherein an oxide layer having athickness of approximately 20,000 Angstroms is formed.

3. The method of improving the breakdown voltage and reverse currentcharacteristics of a silicon PN junction diode, comprising the steps of(a) cleaning the silicon slice in trichloroethylene;

(b) rinsing the slice in methyl alcohol and then in boiling water;

(c) treating the slice with concentrated sulfuric acid and then rinsingit in running water;

(d) soaking the slice in hydrofluoric acid and then rinsing it in water;

(e) boiling the slice in deionized water and then in nitric acid;

(f) rinsing the slice in water; i

(g) oxidizing the slice in steam at about 1200 C.; and

(h) drying and cooling the slice in helium.

4. The method of preparing a protected silicon PN junction whichcomprises the steps of degreasing the silicon slice intrichloroethylene, rinsing it in methyl alcohol and water, boiling itfor minutes at 100 C. in nitric acid, rinsing it in deionized water for5 minutes, heating it in concentrated sulfuric acid at 200 C. for 20minutes and rinsing in deionized running water for 5 minutes, soaking inhydrofluoric acid for 30 minutes and rinsing in water 5 minutes, boilingin deionized water twice for 15 minutes and rinsing in water 5 minutes,boiling in nitric acid 30 minutes and rinsing in deionized running waterfor 30 minutes, oxidizing about five and one-half hours in steam atatmospheric pressure at 1200 (3., and drying in an inert atmosphere forone hour.

5. The method of improving the breakdown voltage and reverse currentcharacteristics of a silicon PN junction diode, comprising the steps of:

(a) cleaning the silicon slice in trichloroethylene;

(b) rinsing the slice in methyl alcohol and then in boiling water;

(c) treating the slice with concentrated sulfuric acid and then rinsingit in running water;

(d) soaking the slice in hydrofluoric acid and then rinsing it in water;

(e) boiling the slice in deionized water and then in nitric acid: (f)rinsing the slice in water;

(g) oxidizing the slice in steam at about 1200 C.;

and

(h) drying and cooling the slice in an inert gas.

6. In a method of treating a semiconductor device of the type comprisinga semiconductor wafer having a P-N junction therein, such treatmentbeing for the purpose of improving the breakdown voltage and reversecurrent characteristics of the P-N junction, the steps of:

(a) exposing the surfaces of the wafer to sulfuric acid;

(b) forming a coating of silicon oxide on the surface of the wafer; and

(c) then cooling in an inert gas.

No references cited.

ALFRED L. LEAVITT, Primary Examinerm W. L. JARVIS, Examiner.

1. THE METHOD OF PREPARING A PROTECTED SILICON PN JUNCTION WHICHCOMPRISES THE STEPS OF DEGREASING THE SILICON SLICE INTRICHLOROETHYLENE, RINSING IT IN METHYL ALCOHOL AND WATER, BOILING ITFOR 20 MINUTES AT 100*C. IN NITRIC ACID, RINSING IT IN DEIONIZED WATERFOR 5 MINUTES, HEATING IT IN CONCENTRATED SULFURIC ACID AT 200*C. FOR 20MINUTES AND RINSING IN DEIONIZED RUNNING WATER FOR 5 MINUTES, SOAKING INHYDROFLUORIC ACID FOR 30 MINUTES AND RINSING IN WATER 5 MINUTES, BOILINGIN DEIONIZED WATER TWICE FOR 15 MINUTES AND RINSING IN WATER 5 MINUTES,BOILING IN NITRIC ACID 30 MINUTES AND RINSING IN DEIONIZED RUNNING WATERFOR 30 MINUTES, OXIDIZING ABOUT FIVE AND ONEHALF HOURS IN STREAM ATATMOSPHERIC PRESSURE AT 1200*C., DRYING IN HELIUM FOR ONE HOUR ANDCOOLING TO ROOM TEMPERATURE.